Cocaine is so addictive that it stands apart from other drugs and deserves special study. Chronic cocaine abuse is associated with changes in the central nervous system (CNS) in humans. For example, the results of neurochemical, neurobehavioral, and neuroimaging studies report that chronic cocaine abuse is selectively associated with abnormalities in the limbic prefrontal cortex. This study will evaluate abstinent cocaine abusers: 1) to determine the magnitude and specific patterns of the neurobehavioral performance in chronic cocaine abuse and relate these to measures of regional cerebral blood flow (rCBF) and brain structure; 2) to determine if chronic cocaine abuse relates to abnormalities of rCBF in orbitofrontal and anterior cingulate cortices during cognitive activation; 3) to determine if chronic cocaine abuse is associated with structural deficits (i.e., smaller brain volumes) in different regions of the prefrontal lobe. Three groups each with 25 participants will be studied. Group 1 (cocaine abusers) will meet DSM IV criteria for cocaine dependence or abuse; Group 2 (polysubstance abusers) will be dependent on or abuse drugs other than cocaine; and Group 3 will consist of a non-drug using control group. Groups will include men and women, aged 20-45 years. They will be matched for age, sex, race, socioeconomic status (SES), and maternal education. The drug using groups will stay for 30 days on the Clinical Inpatient Research Unit, NIDA-IRP. All testing in Groups 1 and 2 will occur in the 4th week to minimize the effects of both physical and psychological withdrawal from drugs and alcohol. Group 3 (controls) will undergo testing during a 7-day stay in the Johns Hopkins Bayview General Clinical Research Center. Neurobehavioral characteristics will be assessed using neuropsychological tests and questionnaires that evaluate history of drug use, drug craving, personality characteristics, and neuropsychiatric symptoms. Physiological and neuroanatomical characteristics will be determined with rCBF activation, positron emission tomography (PET) and brain magnetic resonance imaging (MRI).